It is well known in the art of electrographic printing and electrophotographic copying to form an electrostatic latent image corresponding to either the original to be copied, or corresponding to digitized data describing an electronically available image.
In electrophotography an electrostatic latent image is formed by the steps of uniformly charging a photoconductive member and imagewise discharging it by an imagewise modulated photo-exposure.
In electrography an electrostatic latent image is formed by imagewise depositing electrically charged particles, e.g. from electron beam or ionized gets onto a dielectric substrate.
The obtained latent images are developed, i.e. converted into visible images by selectively depositing thereon light absorbing particles, called toner particles, which usually are triboelectrically charged.
In magnetography a latent magnetic image is formed in a magnetizable substrate by a patternwise modulated magnetic field. The magnetizable substrate must accept and hold the magnetic field pattern required for toner development which proceeds with magnetically attractable toner particles.
In toner development of latent electrostatic images two techniques have been applied: "dry" powder and "liquid" dispersion development of which dry powder development is nowadays most frequently used.
In dry development the application of dry toner powder to the substrate carrying the latent electrostatic image may be carried out by different methods known as, "cascade", "magnetic brush", "powder cloud", "impression" or "transfer" development also known as "touchdown" development described e.g. by Thomas L. Thourson in IEEE Transactions on Electronic Devices, Vol. ED-19, No. 4, April 1972, pp.495-511.
The visible image of electrostatically or magnetically attracted toner particles is not permanent and has to be fixed by causing the toner particles to adhere to each other and the substrate by softening or fusing them followed by cooling. Normally fixing proceeds on more or less porous paper by causing or forcing the softened or fused toner mass to penetrate into the surface irregularities of the paper.
Dry-development toners essentially comprise a thermoplastic binder consisting of a thermoplastic resin or mixture of resins (ref. e.g. U.S. Pat. No. 4,271,249) including colouring matter, e.g. carbon black or finely dispersed dye pigments. The triboelectrically chargeability is defined by said substances and may be modified with a charge controlling agent.
There are different types of fusing processes used for fusing a toner powder image to its support. Some are based upon fixation primarily on fusing by heat, others are based on softening by solvent vapours, or by the application of cold flow at high pressure in ambient conditions of temperature. In the fusing processes based on heat, two major types should be considered, the "non-contact" fusing process and the "contact" fusing process. In the non-contact fusing process there is no direct contact of the toner image with a solid heating body. Such process includes: (1) an oven heating process in which heat is applied to the toner image by hot air over a wide portion of the support sheet, (2) a radiant heating process in which heat is supplied by infrared and/or visible light absorbed in the toner, the light source being e.g. an infrared lamp or flash lamp.
According to a particular embodiment of "non-contact" fusing the heat reaches the non-fixed toner image through its substrate by contacting the support at its side remote from the toner image with a hot body, e.g. hot metallic roller.
In an embodiment of common "contact" fusing the support carrying the non-fixed toner image is conveyed through the nip formed by a heating roller also called fuser roller and another roller backing the support and functioning as pressure exerting roller, called pressure roller. This roller may be heated to some extent so as to avoid strong loss of heat within the copying cycle.
The last mentioned fusing process has been employed widely in low-speed as well as high-speed fusing systems, since a remarkably high thermal efficiency is obtained because the surface of the heating roller is pressed against the toner image surface of the sheet to be fixed.
This fusing system has to be monitored carrefully in that when the fuser roller provides too much thermal energy to the toner and paper, the toner will melt to a point where its melt cohesion and melt viscosity is so low that "splitting" will occur, and some of the toner is transferred to the fuser roller wherefrom the toner stain may be transferred in a next copying cycle on a copy sheet whereon it may not deposit; such phenomenon is called "hot offset", and requires appropriate cleaning.
When on the other hand too little thermal energy is provided toner particles will adhere not Strong enough to the initially cold paper but can already stick sufficiently to the fuser roller wherefrom the "unfixed" partially fused toner particles will likewise be deposited onto the copy sheet of the next copying cycle, resulting in what is called "cold offset".
In order to avoid these phenomena the toner used in said fixing system operating with a hot pressure-fuser roller has to be composed such that said offset is minimized, but even then an external release agent, wetting the fuser roller has to be used. The application of an external liquid release agent represents an extra consumable and requires apparatus adaption making it more expensive. The release agent will inevitably also transfer to the copy paper and may produce prints having a fatty touch. The use of internal release agents, e.g. waxy polyolefine compounds, may after a certain period of use cause smearing to carrier particles and change triboelectric properties.
Moreover, for producing graphic art quality prints, e.g. of screened images, toner-contacting pressure fuser rollers will distort the dot structure of the screened images. Such will be particularly the case when the pressure-fuser roller has no perfect smooth structure and texturizes the obtained image.
The non-contact fusing process has not these drawbacks but requires for optimal fixing toners that have a visco-elastic behaviour such that the toner particles in the absence of pressure and by moderate heat fuse together and still form on cooling a toner mass of sufficient hardness to avoid smearing by contact in machine or manual use of the copies.
A toner with a too soft nature can give rise to problems as e.g. (i) smearing of the toner on the photoconductor layer, (ii) smearing of toner on the carrier particles, (iii) agglomeration of the toner with itself and the carrier particles when present whereby blocking and transport problems may arise and image resolution will be impaired.
When non-contact fusing systems are used in colour printing devices applying differently coloured toners deposited in succession it is important that the separate toner images become co-fused strongly enough with each other.
Such brings about that in non-contact fusing the melting point of the toner mass and consequently also of the thermoplastic binder(s) of the toner has to be much lower than of the toners applied in hot pressure-roller fixing.
Such will result in two conflicting requirements for a toner binder applied in non-contact fusing systems, particularly radiant systems, in that sufficiently low melt viscosity has to be combined with good toner hardness. For coloured toners the absorption of radiant energy and conversion in conduction heat will be dictated by the kind and amount of colorant and require an adapted melt viscosity that will be lower according as the absorption is less, which will be particularly low when a colourless toner is used that still may absorb invisible infrared radiation.